Nance Molloy's cheerful disposition, in sharp contrast to the slum near the cathedral where she and her parents make their home, earns her the nickname "Sunshine Nan." However, her fights with the cathedral choirboys, led by MacPherson Clark, gets her into trouble. For interfering in a family squabble, she and her pal, Dan Lewis, are sent to reform school for five years, and upon her release, she becomes a stenographer in the elder Clark's shoe factory. Dan meanwhile has taken a job in the factory's chemical department, where he invents a dye process that promises to make him rich. In trouble over a woman, MacPherson decides to steal the formula and present it to his father as his own. Nan enters the lab just as MacPherson is copying Dan's formula, and the two old enemies begin to fight. Dan joins in the scuffle, but when some chemicals spill, the lab is enveloped in fire. Dan rescues Nan, and MacPherson, awakening in the hospital, remorsefully confesses his crime.